The present invention relates to a process and an apparatus for the modulation and amplification of light beams.
It more particularly applies to the processing of optical signals for optical telecommunications, for "intra-chip" and "inter-chip" optical interconnections and for neuronal networks.
Multilayer structure photon devices with perpendicular access to the layers and which are known as "vertical structures", constitute one of the most appropriate technical approaches for utilizing parallelism and for taking optimum advantage of the very high flow rates allowed by optical links.
A new class of components based on the use of a vertical Fabry-Perot resonator or cavity has recently been developed in III-V semiconductors for producing:
lasers (cf. documents (1) and (2) which, like the other documents referred to hereinafter, are listed at the end of the description), PA1 optical thyristors (cf. document (3)), PA1 bistable devices (cf. document (4)), PA1 modulators (cf. documents (5) and (6)) and PA1 optical gates (cf. document (6)). PA1 1. When it is used with normal incidence, the apparatus according to the invention is insensitive to the polarization or biases. This is particularly important for modulating signals, which have been either transmitted by an optical fibre and whose polarization bias state generally varies over a period of time, or emitted by a strongly polarized laser source. PA1 2. The apparatus according to the invention has a Fabry-Perot resonator or cavity, which it was sought to avoid in known amplifier-modulators by depositing thereon antireflection layers. Due to the existence of said Fabry-Perot cavity or resonator in the apparatus according to the invention, the amplifying character of the active medium thereof can give rise to a laser emission, which is utilized in certain embodiments of the apparatus. PA1 3. The light propagation in the apparatus according to the present invention is very different to the light propagation in known amplifier-modulators. Thus, in such a known apparatus, the light is propagated in a waveguide, whose axis is parallel to the substrate on which the apparatus is formed. This waveguide is generally transverse monomodal, so that the output beam can only have a single direction. PA1 4. The size and the "vertical" structure of the microresonators according to the invention are essentials for obtaining bidimensional arrays with a very high integration density, which cannot be the case with the aforementioned, known amplifier-modulators.
In most of these structures, the front and rear mirrors of the cavity or resonator are constituted by stacks of periodic "quarter wave" multilayers so as to obtain the optimum reflectivity necessary for the operation of such devices.
The majority of known modulators of this type make use of the electroabsorption effect linked with the Stark effect in multiple quantum wells, or the Wannier-Stark effect in superlattices, namely the optical absorption variations induced by an electric field. Thus, they are electric control modulators (cf. document (5)).
"All-optical" Fabry-Perot multilayer structure modulators are also known, in which the optical absorption variations are induced by an optical control beam (cf. document (6)).
Modulators are also known, which integrate a Fabry-Perot multilayer structure and a heterojunction phototransistor able to switch an optical beam by means of another lower intensity optical beam, the gain being supplied by the phototransistor (cf. document (7)).
The modulators which at present have the best performance characteristics from the standpoint of contrast between the "off" state and the "on" state are those which take advantage of the combination of the variations of the electrically or optically induced optical absorption and the resonances in a Fabry-Perot cavity surrounding an active medium.
The characteristics of the resonator or cavity (reflectivity of the front and rear mirrors and the size of the resonator or cavity) are calculated so that the reflectivity in the "off" state is virtually zero. In the "on" state, the reflectivity is not zero, but never reaches 100%, which leads to a prohibitive attenuation of the beam to be modulated, also known as "insertion loss", introduced by the modulator.